The effectiveness of a safety belt in protecting the occupant of a vehicle from being thrown against interior parts of the vehicle during a collision may be improved by tightening the belt (within tolerable limits) at the moment that the restraining force of the belt is needed. Safety belt tension devices are known which draw the safety belt tightly against the body of the vehicle occupant in an impending collision to eliminate the belt slack occurring in service due to the relatively loose application of the belt under normal conditions. U.S. Pat. No. 3,871,470, issued Mar. 18, 1975, discloses a safety belt tension device provided with a piston movable within a cylinder, a selectively releasable pressure medium for driving the piston within the cylinder, a sensor for releasing the medium at the appropriate time, a connecting rod for translating the movement of the piston into a tensioning of the safety belt, and a return stop for maintaining the increased safety belt tension, once applied, to prevent the vehicle occupant from being thrown forward in the event of continued deceleration. The return stop is especially important in accidents in which the vehicle undergoes several successive impacts, as may occur, for example, if the vehicle skids against a series of obstacles or turns over. In the patent mentioned above, the return stop operates directly on the piston connecting rod by means of gripping jaws, arranged concentrically about the connecting rod, having an outer diameter tapering conically inward in the direction of locking and surrounded by jaw closing elements, arranged in the cylinder, having an inner diameter matching the contour of the gripping jaws.
The drawback of this known device is that the elements of the return stop mechanism must be precisely dimensioned and finished to within very close tolerances, thereby necessitating considerable expense in fabrication and assembly.